Upgrade ESLint from v8 to v9/10 for official Node 24 support

[Tim020]

Problem

ESLint 8.57.0 does not officially support Node 24. While it works in practice, we should upgrade to ESLint 9 or 10 for official support.

Breaking Changes

ESLint 9+ introduces the flat config system, which is a major breaking change:

• .eslintrc.js → eslint.config.js
• Different plugin registration syntax
• Different extends/configs pattern
• All existing rules need verification

Implementation Steps

• Research ESLint flat config migration guide
• Create new eslint.config.js following flat config format
• Migrate all existing rules and plugins
• Update package.json dependencies (eslint, plugins)
• Test linting locally
• Update CI workflows if needed
• Remove old .eslintrc.js
• Update documentation

References

• ESLint v9 Migration Guide
• Flat Config Documentation

Estimated Effort

2-3 hours (config migration requires careful testing)